1. Field of the Invention
The present invention relates to the method of making a semiconductor device of the type integrated with a memory array of a high voltage operation system which requires relatively thick oxide films for separation of individual memory elements (hereinafter, referred to as "thick field oxide film") so as to withstand against an applied high operating voltage, and integrated with a peripheral circuit of a low voltage operation system for driving the memory elements, which needs relatively thin oxide films for separation of individual components thereof (hereinafter, referred to as "thin field oxide film") so as to increase the integration density thereof. More specifically, the present invention relates to the method of forming thick and thin oxide films concurrently on a common semiconductor substrate of, for example, a highly integrated semiconductor nonvolatile memory device of the type described above.
2. Prior Art
The conventional method of forming field oxide films will be briefly explained with reference to FIGS. 2a-2c which show a process of forming field oxide films according to the conventional selective oxidization method (hereinafter, referred to as "LOCOS").
Referring to FIG. 2a, firstly a major surface of semiconductor substrate 1 is thermally oxidized to form an under-layer oxide film 2 (hereinafter, referred to as "PAD film").
Next referring to FIG. 2b, a nitride film 3 is formed on the PAD film 2 according to chemical-vapor-deposition (hereinafter, referred to as "CVD"), and then selectively etched according to a photolithographic process to form a mask pattern for use in the LOCOS.
Lastly, referring to FIG. 2c, the substrate 1 is subjected to thermal oxidization to form a field oxide film 7.
However, as described above, according to the conventional technology, the thickness of formed field oxide film 7 is uniform throughout the surface of substrate. Such conventional technology can not be practical to selectively form thick field oxide films on the memory array region of a semiconductor nonvolatile memory device and thin field oxide films on the peripheral circuit region of the same semiconductor nonvolatile memory device. Consequently, according to the conventional technology, the thick field oxide films would be formed on the peripheral circuit region as well as on the memory array region to thereby sacrifice the integration density of the peripheral circuit region while maintaining the electrical separation of memory elements with the thick field oxide film against the high operation voltage applied to the memory element.